Method and device for fixing a joint prosthesis

ABSTRACT

The invention comprises a method and a device for fixing a joint prosthesis (1, 2; 5) in which all surfaces intended for tissue contact consist of titanium or a titanium-based tissue-compatible metallic material. For safely anchoring the prosthesis in relation to the bone tissue, use is made of pointed fixing elements (3, 11,16) which from the side of the bone tissue that should be facing the prosthesis are driven into the bone tissue with their pointed portions, and which are anchored in cement (9) between the prosthesis and the bone tissue or directly to the prosthesis, such that a solid connection is established by the bone tissue growing onto the anchored pointed portions. For proper adjustment of the prosthetic joint cup and the prosthetic joint ball, these members are moulded under pressure and heat, each respective articular surface being shaped against a model mould of the mating articular surface, and are hardened or sintered in shape contact for minimizing any aftertreatment.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a device for promoting the connectionand fixing of joint prostheses, in particular hip joint prostheses butalso shoulder, knee, foot and finger joint prostheses etc.

2. Description of Prior Art Practices

The implantation of joint prostheses is today general routine withinorthopedic surgery. The surgical technique generally faces surmountabledifficulties. However, a serious problem, apart from infections andthrombosis, is that one or both of the prosthetic components may loosen.The most typical joint reconstruction is the so-called hip jointreconstruction. About 5,500 operations of this type were performed inSweden in 1981 and the commonest reasons therefor is arthosis in the hipjoint or joint injuries caused by bone fractures and malformationsresulting from other causes, e.g. rheumatic affections.

The problems involved in hip joint reconstruction have been subjected tointensive studies over the last decades and were discussed, for exampleat the so-called Consensus conference in Stockholm on May 12-14, 1982.In a Consensus statement made by the Medical Research Council after theconference, it was recommended in hip joint arthroplasty to use afemoral part of metal and pelvic part of plastic, and in routine casesto use cement for fixing the prosthesis.

From other sources it appears that by "cement" is generally understood apolymer which is allowed to polymerize in situ. The polymerizationtemperature may vary according to the components used but oftenconsiderably exceeds 47° C. which is a limit for the temperature towhich bone tissue can be exposed without any commencing injury to thetissue resulting in so-called membrane formation.

Many researchers (Rik Huiskis at the Orthopedic Institute of theUniversity of Nijmegen, E. Morscher at the Institute of OrthopedicSurgery at the University of Basel and others) are of the opinion thatthe material for the femoral part of the prosthesis should be selectedamong stainless steel, cobalt-chromium alloys and titanium, and thatplastic should be selected for the pelvic part of the prosthesis. Thiscombination is advantageous in that it entails relatively low frictionand is wear-resistant. The choice of material is dictated not only bythe mechanical properties of the material but also by the requirementthat the material should be biocompatible with the body tissues. The useof stainless steel has declined to an increasing extent, the majorinterest being today directed toward titanium, titanium alloys andcobalt-chromium alloys. Of the metals hitherto tested, titanium seems tobe the most tissue-compatible and corrosion resistant, and it has theexcellent property of being highly resistant to fatigue and isrelatively flexible.

As material for the acetabular cup, researchers primarily recommendpolyethylene, Delrin and methyl methacrylate. High-density polyethyleneis extensively used for acetabular cups because of its excellentfrictional and wear resistant properties in combination with theabove-mentioned metals and in that fragments resulting from wear cause aminimum of irritation to the tissue. Delrin, which has lately come intouse, is harder than polyethylene and would scarcely offer any advantagesover polyethylene. Among other possible materials, mention may be madeof ceramic materials with high tissue compatibility. Such materialsresist corrosion but are considered unreliable in respect of mechanicalproperties and may cause irritation and infections by fragmentsloosening from the surfaces of the ceramic material.

For fixing the parts of the prosthesis, and in particular the prostheticpart in the femur, use is made of cement which serves to fill the entirespace between the prosthesis and the inner side of the bone wall and todistribute the load over as large an area of the bone as possible.Cement materials hitherto used (plastic glues) ensure good adherence tometal but are inapt to establish a chemical bonding to bone tissue. Themain function of the cement therefore is to form a stabilizing fillingbetween the prosthesis and the bone.

Although the prosthetic materials for the acetabular cup and the femoralhead with the stem have been thoroughly tested and give satisfactoryresults in respect of the friction produced between the prosthetic cupand the prosthetic head or ball, and although materials with suitableelastic properties may be selected, one serious problem remains, i.e.that micromovements occur between the wall of the femoral medullarycanal and the stem of the femoral prosthetic part fixed therein becauseof the loads on the femoral joint head caused by the weight and themovement of the body and because the stem is not sufficiently stablyfixed with respect to the bone tissue in the wall of the medullarycanal. Such micromovements which will be most pronounced between theupper and lower ends of the stem too often result in that the stemloosens, thus necessitating reoperation.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a system and a methodwhich ensure safe and permanent anchorage of prosthetic parts of theabove-indicated materials having good tissue-compatible and mechanicalproperties versus bone tissue without adding any undesirable properties.

A further object of the invention is to provide a simplified method foreconomic production of joint cup and joint head prostheses withperfectly mating articular surfaces of low friction and suitable elasticproperties with respect to each other.

According to the invention, these objects have now been achieved by adevice and a method having the features stated in the accompanyingclaims.

BRIEF DESCRIPTION OF THE FIGURE OF DRAWING

The invention will be described in greater detail hereinbelow withreference to the accompanying drawing which shows an acetabular cupprosthesis which is designed in accordance with the invention and fixedto the pelvis by fixing means and by a method for fixing prosthesesaccording to the invention, and which, in longitudinal section, showspart of the femur with a prosthesis fixed in the femoral medullary canaland comprising a joint head or ball with a stem conformed to theacetabular cup of the joint.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The acetabular cup prosthesis shown in the drawing consists of acup-shaped main portion 1 and a layer 2 applied to the concave side ofthe cup and forming the articular surface of the acetabular prosthesis.The main portion 1 of the acetabular prosthesis is preferably obtainedby compression moulding a metal powder and sintering the metal powderaccording to known powder moulding, compression moulding and sinteringmethods, such that its concave side is provided with pores. However, itmay also be possible to obtain the portion 1 from metal by a suitablecutting technique or a conventional metal shaping technique, such thatthe concave side is given an uneven structure corresponding to theabove-mentioned pores and ensuring good adherence to the layer 2 whenprepared as described below. A suitable material for the acetabularportion 1 is selected from any of the metals mentioned by way ofintroduction, preferably titanium which is known for its tissuecompatibility, a tissue-compatible titanium alloy or optionally acobalt-chromium alloy of any other tissue-compatible metallic material.

The cup portion 1 is preferably designed in such a manner that itsconvex upper face intended to engage the pelvis B is also porous orrough to increase the ability of ongrowth of the bone tissue to saidsurface.

The acetabular portion 1 can be fixed to the pelvis, for instance bymeans of pins or screws in a manner which is known in conjunction withthe fixing of cup prostheses of plastic, but the cup portion 1 ispreferably fixed in accordance with the invention in such a manner thata plurality of pins 3 projecting from the convex side of the portion 1and consisting of a tissue-compatible metal onto which living bonetissue can grow, are fixed in the material. These pins should have barbsand pointed ends, such that they can be driven into the underlyingpelvic bone and mechanically hook onto the bone tissue so as to form abond which will be subsequently strengthened by the bone growing ontothe pins.

Before the acetabular portion 1 is connected to the pelvic bone, thelayer 2 is preferably applied according to the invention in such amanner that the cup portion 1 is placed in an injection moulding tool bymeans of which the layer 2 is injection moulded against a model of thejoint ball so as to obtain a good calibration of the articular surfaceof the joint cup with respect to a joint ball with which it is tocooperate. A suitable material for the layer 2 is preferably selectedamong any of the initially mentioned plastics or any othertissue-compatible plastic with suitable elasticity and thickness fordistributing the loads which generally occur in a hip joint. Suitableplastics are for example plastics of the types polyethylene, Delrin andmethyl methacrylate. Instead of plastic, it is possible to choose asuitable enamel or a ceramic material.

The femoral prosthetic part 5 is preferably prepared in the same way andfrom the same types of material as the acetabular prosthetic portion 1.By preparing the prosthetic part 5 by moulding, compressing andsintering a metal powder of a suitable particle size, it is possible toobtain an appropriately porous or rough surface to be connected tocement of the type which is generally used for fixing prosthetic stemsin the medullary canal of a bone. However, the femoral prosthetic partmay also be produced in a conventional way. In this case, too, thesurface of the stem portion should be sufficiently porous or rough foradequate adherence to the cement material selected for applying in themedullary canal of the bone.

When the prosthetic part 5 has been prepared, it is inserted in aninjection moulding tool by means of which there is applied to the jointball 6 a layer 7 of a material which gives a uniform, smooth articularsurface similar to the articular surface of the, acetabular prosthesis1, 2. The injection-moulded material on the joint ball is selected withregard to the injection-moulded material on the joint cup or socket soas to obtain the best possible combination in respect of both frictionand elasticity between the joint socket and the joint ball, for instanceplastic to plastic, enamel to plastic or ceramic to plastic. Byinjection moulding the layer 7 on a model of the joint socket and viceversa, it is possible to obtain a very accurate fit between socket andball. By this moulding technique for both the joint ball and the jointsocket, the very expensive finishing and polishing operations hithertoused may be reduced or entirely dispensed with.

However, for the combination of articular surfaces of the type metal toplastic it is also possible to apply a metallic surface to the jointball by metal evaporation in vacuum or, possibly, by electrolytic orelectrochemical metal deposition. However, these methods generallynecessitate the use of more precise manufacturing methods for the jointball surface to which the layer 7 should be applied, because it is moredifficult to achieve the required exact calibration of shapes anddimensions by plating or metal deposition than by injection moulding ofan injection mouldable material. It should be emphasized that asubstantial advantage of the manufacturing method according to theinvention is that it makes it possible to reduce the demand for accuracyin the manufacture of the prosthetic parts on which the articularsurfaces are to be designed by applying a layer of material.

As appears from the above, the acetabular cup. prosthesis is fixed tothe pelvic bone without the use of cement. For fixing the stem 8 of theprosthetic part 5 in the femoral medullary canal, use is however made ofthe well-tried technique of embedding the stem in cement 9 filling theentire space between the stem 8 and the wall 10 of the bone. As appearsfrom the introduction to the specification, this method does however notgive a fully satisfactory result because the cement may loosen from thebone tissue. Therefore, the application of cement is supplementedaccording to the invention by fixing the cement material to the bonetissue by means of elements of a metallic material which istissue-compatible and allows the bone tissue to grow onto the elementsand which is "cement-compatible", such that the conventional cementmaterial used will adhere to the elements.

In accordance with the invention, the metallic fixing elements used arepreferably barbed pins 11 similar to the pins 3 which are used forfixing the joint cup to the pelvic bone. The barbed pins 11 can beapplied to the bone tissue, for instance by driving them into the bonetissue from the inner side of the medullary canal by means of a drivingtool or by pressing them into the bone tissue by means of any suitabletool. The pins are fixed in place before the stem of the femoralprosthetic part is inserted and cemented in the medullary canal of thebone. The pins should only penetrate into the bone tissue and should ofcourse not project from the outer side of the wall of the bone. On theother hand, the pins should have, at the inner side of the medullarycanal, sufficiently large contact surfaces, contact heads or projectionsto offer an adequate attachment surface for the cement material.

Instead of barbed pins, which are preferred, use may be made of metalscrews which may optionally be screwed from the outside so as to extendinto the medullary canal of the bone to be enclosed therein and bondedto the cement material.

As an alternative of more or less sophisticated pins, use may be made ofmetal plates or strips which are provided with pins and to which thecement material to be used for cementing the prosthetic stem adheres.

It may be advantageous to use pins, screws or the like having poroussurfaces for good adherence to the cement material and for stableongrowth of the bone tissue to the pins, screws etc.

It is advantageous to design the metal elements 11 in such a manner thatthe cement material 9 not only adheres to the metal elements but alsowill be firmly connected, because of the shape of such elements, byembedding portions thereof. A sufficient number of relatively smallpins, screws etc. should be used, especially in the region of the lowerend and neck portion of the prosthetic stem, such that there will be asufficient number of fixing points to efficiently secure the cement tothe wall of the bone.

By using a certain concentration of pins, nails etc. in annular areas inthe upper portion of the medullary canal of the bone adjacent the jointball and at the lower end of the prosthetic stem, it is possible inthese areas to obtain concentrated anchorage, and the use of a centeringring of metal which, according to conventional methods, is often mountedon the stem, especially at the lower end thereof, may be dispensed with.

The number of such fixing points should however be selected also inconsideration of the risk that a reoperation may become necessarydespite the reliable fixing. In a reoperation, the fixing points must ofcourse be loosened to make it possible to remove the prosthesis, whichspeaks for as few fixing points as possible. For loosening the pins inconnection with a reoperation, it may become necessary, from theoutside, to bore or otherwise break loose the attachments. The looseningof the fixing points will of course become easier, the fewer the fixingpoints are. If the pins are bored loose, the holes which are left openwill be so small that they will be fully healed by growth of bone tissueafter a reoperation.

As already intimated above, use is often made of a guide or centeringmeans at the outer (lower) end of the stem of the femoral prosthesis tomaintain the stem in as correct a position as possible during thecementing operation. In a per se known embodiment, the centering meansconsists of a band-shaped ring disposed on the stem adjacent its lowerend and being of the same material as the stem. The ring generallycarries a sparse corona of obliquely outwardly-upwardly directedresilient tongues or tines of the same material which are urged by thewall of the bone inwardly towards the prosthetic stem during theinsertion thereof in the medullary canal. The ring with its resilienttongues or tines are capable but to an insignificant extent to preventthe loads on the femoral prosthesis from initiating a vibratory andoscillatory movement of the stem in the medullary canal. Such anoscillatory movement, which may initially be microscopically small andhas its amplitude at the lower end of the stem, tends to increase withtime and may in a relatively short time have a devastating consequencefor the retention of the femoral prosthesis.

To pursue the idea of stabilizing the prosthesis in the femoralmedullary canal, it is therefore suggested according to the inventionthat a guide or centering means, at least at the lower end of theprosthetic stem, is fixed by means of hooks, pins or the like whichpenetrate the wall of the bone and consist of titanium or atitanium-based tissue-compatible material allowing the bone tissue togrow onto the pins or the like.

There are many different ways for achieving such a penetration of pinsor like elements for ongrowth of bone tissue, which are fixed or beingfixed to a guide or centering means of the above type or simply to theprosthetic stem itself,

One way is implanting pins from the outside in the wall of the bone andfixing the pins to the stem. Another way is fixing a ring by means ofpins or a circle of pins in the bone prior to the insertion of theprosthesis, such that the prosthetic stem can thereafter be inserted inthe ring or circle of pins with adequate fit therein for stableretention with or without the use of cement. Another possible wayaccording to the invention, which seems promising but has not yet beenclinically verified, is using a pin-equipped, expandable annular member12 which is expandable from the outside and, for example, is of theschematic design intimated in the drawing.

The illustrated annular member 12 is expandable by means of a bolt 13which with a threaded end portion 13' engages in two internally threadedexpansion washers 14 on either side of an expandable annular torus 15which is mounted at the end portion of the stem 8 and from which anumber of pins project radially outwardly, so that they can be urgedinto the femoral wall 10 by rotation of the bolt 13. The bolt extendsthrough a channel bored in the stem 8 to a point at the upper endportion of the stem that is spaced apart and laterally offset from thejoint ball 6. At its upper end, the bolt 13 has a member 17 with which atool (not shown) can engage.

For moving the expansion washers 14 towards each other for expanding themember 15, one washer has a left-hand thread and the other a right-handthread, and the thread of the bolt consists of one left-hand threadedportion and one right-hand threaded portion, i.e. the washers 14 and thebolt 13 act as a turnbuckle device in the illustrated example.

Instead of the expansion assembly 12 shown in the drawing, it ispossible to use any other per se known expandable device suitable forthe contemplated purpose, provided it can properly stabilize the stem 8in the above-described manner by expanding, such that the pins willpenetrate into the femoral wall, and/or by expanding into engagementwith and abutment against the inner side of the wall.

The toroid member 15 should consist of a tissue-compatible material,such as titanium or a titanium-based material or a tissue-compatibleplastic. It is possible by a suitable choice of material and design toachieve a certain elasticity to force the expandable member 12 (15) toconform to the shape of the medullary canal without neglecting the needfor eliminating vibrations or at east reducing them in the prostheticstem in the femoral medullary canal.

It should be noted in particular that the expandable annular member andthe prosthetic stem can be so arranged that the stem can be inserted inthe annular member after expansion thereof in the medullary canal beforethe prosthesis is inserted in place. In this case, the prosthetic stemneed not have a bore and the bolt 13 may be dispensed with, but therewill instead be required a tool which can be inserted through the boneor at the end of the medullary canal for expanding said member.

It should be noted that the invention is only schematically illustratedin the drawing, in which for instance the thickness of the cement layer9 and of the layers 2 and 7 forming articular surfaces is chosen ratherfor purposes of illustration, which also applies to the pins serving asfixing elements.

The thickness of the different layers and in particular the geometricalshapes of the fixing elements may vary and several modifications of theshape of the fixing elements are possible within the scope of theinvention. Also, it is possible to use many combinations of materialsfor the fixing elements. Thus, it is possible, for instance, to make thefixing elements of steel or any other inexpensive material and to coatthe surface with a more biocompatible material, primarily pure titaniumor a titanium-based material which is particularly tissue-compatible andin other respects biologically acceptable.

It should however be emphasized that the femoral prosthesis need notconsist of titanium in its entirety. On the contrary, an embodiment ispreferred which consists of a core of for instance steel coated with atitanium metal. The different fixing elements of metal preferablyconsist of for instance steel with a titanium coating in accordance withthe above. Further, it should be noted that the joint prosthesis of theinvention can also be used for joints other than hip joints, forinstance knee joints, shoulder joints etc., and that it is possible, asa supplement to the illustrated fixing elements, to use metal screwscoated with a surface layer of titanium. Steel screws with a titaniumlayer provide the same tissue compatibility as titanium screws but areless expensive and, moreover, a suitable elasticity is more readilyobtainable by a combination of steel/titanium surface layers.Incidentially, such screws may also be used for fixing bone shafts incases of complicated fractures or for strengthening the bone shaft wherea joint prosthesis is applied.

Nor is the invention restricted to the method described above where themetallic fixing elements are applied before the prosthetic stem 8 iscemented in the medullary canal. In fact, it is possible, from theoutside, to drive or screw fixing elements through the wall of the boneand into the cement material. In order to obviate any impermissiblestresses, use should then be made of fixing elements provided withthroughholes allowing cement subjected to pressure to escapetherethrough. This method of application can also be carried out aftercementing but before the cement has set.

As a supplement to or in replacement of the abovementioned bridgingelements in the form of pins, staples, screws or the like between bonecement for fixing a prosthesis with respect to a bone tissue, and thebone tissue itself, the following means are suggested which for the sakeof simplicity will be described in connection with a hip jointprosthesis with a stem to be connected to the femoral wall, but may alsoapply to many other cases where a prosthesis should be fixed to bonetissue, possibly also artificial roots or stems of teeth.

In the open medullary canal of the bone, titanium particles are insertedwhich are caused to adhere temporarily or more permanently to the bonetissue. The particle layer may be concentrated or relatively sparsedepending on the field of use and on how concentrated points ofattachment are desired. These particles or grains may be applied in anysuitable manner, for instance advantageously by spraying through anozzle. The particle size may in principle be selected within the rangeof from μm to several mm depending on the size of the prosthesis and thethickness of the bone cement layer. Thus, the titanium particles mayalso be in the form of a very fine power. This powder can be applied bymeans of fine nozzles before or after application of the prosthesis butbefore the application of the bone cement. However, it is quite possibleand, in some cases, even advisable to mix the titanium powder or grainswith the bone cement.

In tests on living animal bone tissues, it has been found that the bonetissue grows onto the grains fixed by means of the bone cement, wherebythe cement which readily adheres to the prosthesis, as when pins etc.are used, will form a solid bridge between the bone tissue and theprosthesis.

The titanium grain or powder material can be applied in the form of amore or less easy flowing paste or thin slurry or, if a fine grainedtitanium powder is used, as a pigment-like substance.

Good results are also anticipated for a method according to theinvention in which titanium grains are introduced as a layer between theprosthesis and the bone tissue, whereupon a binder for fixing theprosthesis and for fixing the grains is injected in the powder layer bymeans of an injection needle or in any other suitable way and in such aneasy flowing state that the binder will fill the voids between thegrains by capillary action.

In the case described above, animal tests have confirmed or given toexpect very favourable results which should also apply to humanapplication.

In combination with the fixing and consolidating methods describedabove, in which use is made of a binder of some kind or also simplybonding by sintering, it may also be possible to use fibres, such astitanium fibres, but also other fibres, such as carbon fibres, areconceivable.

We claim:
 1. A method for fixing a prosthesis in relation to living bonetissue comprising applying between the prosthesis and the bone tissueand in contact with the bone tissue, a plurality of fixing elements of amaterial and a surface structure which allows bone tissue to growthereon, wherein the fixing elements have a first end which is insertedinto the bone and a second end with a contact head or projection thereonto provide an attachment surface for the bone cement; said fixingelements being inserted into the bone so that the projections or contactheads are spaced apart from the prosthesis and the bone and applying abone cement which is capable of adhering to the prosthesis, between thebone tissue and the prosthesis, such that the prosthesis whereby theprojections or contact heads are completely embedded in said bone cementthrough said bone cement and said elements, is connected to the bonetissue.
 2. Method as claimed in claim 1, wherein each element to befixed in contact with living bone tissue comprises a body having acoating disposed thereon for surface contact with the bone tissue saidcoating comprising a tissue-compatible material which permits ongrowthof bone tissue to the element, said coating being connected to the bodyso as to form a surface layer which is mechanically inseparable withrespect to the body and which tightly encloses and covers at least theportion of the body which engages the bone tissue in the position of useof the element.
 3. Method as claimed in claim 1 or 2, wherein saidcoating is a coating of a titanium-based or pure titanium.
 4. Method asclaimed in claim 1, wherein each element is an element with a bodyproduced by moulding and sintering a sinterable material, such that thebody or the portion thereof to which said coating is applied, has poresor other uneven portions, said coating covering said uneven portions,such that said uneven portions are reflected on the outer side of thecoating.
 5. Method as claimed in claim 1, wherein said elements arepointed or barbed fixing elements or are in the form of screws or pinswith projections or heads at the ends opposite to those provided withsaid pointed portions, to be anchored in the bone cement between theprosthesis and the bone tissue.
 6. Method as claimed in claim 5 whereinthe pointed fixing elements are screws or pins comprising a body of ametallic material having a coating of a titanium-based metal or puretitanium applied by metal evaportion on a porous surface of said body.7. Method as claimed in any one of claims 5 or 6, wherein some of saidfixing elements are disposed on a supporting, expandable member anddriven with the pointed portions into the bone tissue by expansion ofthe expandable member.
 8. Method as claimed in claim 7 wherein saidfixing elements comprise a powder or grains of titanium which areapplied to the bone tissue and/or the bone cement such that, thetitanium grains permit ongrowth of bone tissue to the grains and suchthat said grains are connected to the bone cement, whereby the bonetissue and the prosthesis are connected to each other.
 9. Method asclaimed in claim 7, wherein the titanium powder or grains are applied byspraying or painting.
 10. Method as claimed in claim 7, wherein thetitanium powder or grains are applied as a dispersion in the liquid. 11.Method as claimed in claim 8, wherein the titanium powder is applied inthe form of a dispersion in a paste-forming substance in the form of adispersion in a paste-forming substance compatible with bone cement. 12.Method as claimed in claim 8, wherein the titanium powder or grains areapplied in contact with the bone tissue as a dispersed additive in thebone cement.
 13. Method as claimed in claim 8, wherein othertissue-compatible fibres are applied in contact with the bone tissue asa layer disposed between the bone tissue and the prosthesis, and thatthe pores and other interspaces between the titanium grains and/or otherfibres are filled with a liquid bone cement which is thereafter causedto set in contact with the prosthesis.
 14. The method of claim 1,wherein the first end of the fixing element is pointed and said firstend is barbed so that the barbs of the elements engage the bone when thefirst end is inserted into the bone.
 15. The method of claim 14, whereinthe fixing elements are in the form of screws or pins.
 16. A method offixing a hip joint prosthesis, said joint being of the type whichcomprises an acetabular cup prosthetic part and a femoral prostheticpart having a ball and stem; said method comprising:fixing saidacetabular cup into a pelvis by means of pins or screws which extendfrom a convex portion of the cup into the pelvic bone; said cup havingan articular surface layer which is applied to a concave surface of thecup before said cup is fixed to the bone; providing a plurality offixing elements; said fixing elements having a first end for insertioninto the bone tissue and a second end with a contact head or projectionthereon; inserting the first end of the fixing elements into the bonewithin a femoral medullary canal; said elements being inserted into thebone so that the projections or contact heads are spaced apart from thebone; inserting the stem section of the femoral prosthetic part into thefemoral medullary canal containing said fixing elements; said stemhaving a centering means attached at a lower end thereof and saidcentering means having penetrating means for penetrating the wall of thebone to fix said centering means in the femoral medullary canal; andsaid stem being narrower than the medullary canal so that uponinsertion, a space is formed between the stem and the bone whereby saidprojections or said contact heads are spaced apart from the stem and thebone; inserting said penetrating means into the bone to fix thecentering means and attached stem to the bone; cementing the stem in acanal by filling the space between the stem and the bone with bonecement whereby said femoral prosthetic part becomes fixed in the femoralmedullary canal.
 17. The method of claim 16, wherein the centering meanscomprises an expandable annular member expandable by means of a threadedbolt extending through a channel; said channel extending from a topportion of the step to the centering means attached to the lower end ofthe stem; said thread of the bolt consisting of one left-hand threadedportion and one right-hand threaded portion and said bolt engaging twointernally threaded annular expansion washers on either side of anexpandable annular torus of the centering means which is mounted at theend portion of the stem and from which a plurality of pins projectradially outwardly to engage the bone; one of said washers havingright-hand thread and the other having left-hand thread so that thewashers and bolt cooperate as a turnbuckle when the bolt is turned tocause the washers to move towards one another and thereby causingexpansion of the annular torus and penetration of the pins into thebone;said method further comprising the step of turning the bolt tocause penetration of the pins extending from the torus into the bone.18. The method of claim 16, wherein the fixing elements are barbed pinsand said method includes the step of inserting said barbed pins into thebone.